The present invention relates generally to audio dynamics processors such as compressor expanders, limiters and noise reduction systems and more particularly concerns audio processors in which the level or amplitude of an audio signal is dynamically altered in response to a generated voltage control signal.
Surround systems, such as disclosed in my U.S. Pat. No. 7,035,413, use numerous audio level detectors and generate control signals in response to a stereo input signal, but the subjective transparency of such surround systems can be greatly improved.
Below-threshold, expander-type noise gates have been commonly known and used for noise reduction in professional recording applications for many years. One of the most successful noise reduction systems for use with musical instruments, such as guitar, bass and keyboards, is a system commercially known as the HUSH® noise reduction system. The HUSH® system uses a combination of low-level downward expansion and dynamically controlled low-pass filtering.
The operation of the dynamically controlled low-pass filter portion of the HUSH® system is disclosed in my previous U.S. Pat. No. 4,696,044. When the HUSH® system is used to provide noise reduction for instruments such as guitar, the downward expander provides the most important and most audible aspect of the performance of the system. Conversely, when using the HUSH® system with composite music, the dynamic filters provide the most critical aspect of the operation of the system.
One of the most difficult applications for noise reduction is the removal or suppression of the noise that is present in high gain guitar systems. My U.S. Pat. No. 4,881,047 discloses a noise reduction system specifically designed to suppress the gain noise of a high-gain distortion circuit. The system disclosed in the '047 patent will also greatly reduce the amount of audible hum present in a high-gain guitar distortion system by reducing the gain of the preamplifier distortion circuit.
While the above disclosed systems and many of the previously available expander noise reduction systems have provided improvements in audio performance, they fall short of achieving optimal performance under all conditions. The prior art below-threshold expander systems typically provide a preset or, in some cases, user adjustable fixed slope release characteristic. In order to avoid audible distortion of the input signal or serious pumping side effects, it is desirable to have a slow release time constant for the control signal. When using a very fast time constant for the control voltage, excessive ripple in the control signal will modulate the VCA of a dynamics processor, thereby causing audible distortion, pumping or breathing. If the input signal contains low frequency components, a fast time constant can cause modulation of each cycle of the audio signal, thereby causing undesirable and, in some cases, very audible distortion.
While slowing the release time of an expander will improve the above mentioned side effects, a slow release time will also allow the noise floor to momentarily become audible when the input signal stops suddenly. This causes another objectionable side effect in the expander performance.
Making the release time dynamically variable as described in my U.S. Pat. No. 4,881,047 can offer improvements in expander performance. However, while this is an improvement over typical below threshold expanders, the above mentioned shortcomings remain in varying degrees.
While the teachings of the '047 patent are an improvement over the prior art, further improvements in performance have been made by clamping the control signal so that the release time begins at a predefined voltage level, typically at a point equal to the expander threshold, as is described in detail in my U.S. Pat. No. 6,944,305.
Audio expanders typically use some form of level detection that converts the input audio signal to a DC control signal. The generated control signal typically has a predefined release time constant characteristic. When the input level drops below a user adjustable threshold point, downward expansion will begin. The amount of expansion will increase as the input signal continues to drop further below the threshold point. In the prior art systems, the detection circuit will charge a timing capacitor well in excess of the predefined threshold point. The result is that, when the input signal stops abruptly, the prior art expander does not provide any reduction of the input signal until the timing capacitor voltage drops below the preset threshold point. This results in a “dead zone” where the control signal is decreasing but has no affect on the operation of the downward expander.
While the teachings of the '047 patent show one way to improve this problem, this design also suffers as a result of having a dead zone in the release response, a problem which is greatly improved in the teachings of the '305 patent. While the '305 patent teaches major improvements over prior art systems in generating a clamped threshold, dynamically variable attack and release response, the system still has one major shortcoming in that a large amount of ripple in the final control signal output can still cause audible audio modulation.
Considering the above, it is apparent that, while presently known solutions to audio distortion have been advancing the state of the art as to specific components of the overall problem, each advance is to some degree mitigated by a concomitant adverse effect on one or more of the other components, a two steps forward, one step back approach to audio distortion.
It is, therefore, an object of the present invention to provide an audio dynamics processing control signal which greatly reduces any audible modulation side effects in the audio signal. It is a further object of this invention to provide an audio dynamics processing control signal which reduces or eliminates the ripple in the fast time constant output in passing through to the final output control signal. And it is a further object of this invention to provide an audio dynamics processing control signal which minimizes or eliminates the introduction of adverse audio distortion effects to a system as an acceptable compromise.